So, I have Sonar 8 and also a Digidesign Mbox 2 audio interface. However I can't seem to get the interface working with Sonar. My guitar is plugged into the interface, and I can hear it through the headphones, but in Sonar I can't find anywhere the option to use the interface as a track input.

Anyone has any experience with Sonar and audio interfaces? Any help at all is appreciated.

I use Sonar occasionally, and with any USB device it's selected under MME devices.

Go into Options > Audio > Advanced and under playback & recording select the driver mode to MME devices, then go to the Drivers tab at the top and check your Mbox is available in there.

Then on the audio track, under the input menu, it should be in the MME devices list.

If that's no good then I have no idea sorry
Have you ever installed ProTools? The Mbox is designed to work with that, and the drivers for it probably come with PT.

I don't think they ever made drivers for Vista though, just XP.
This Mbox did come with Pro Tools over a year ago, but it was installed in my brother's computer. I tried installing Pro Tools on my PC earlier today, but the activation code just wasn't accepted, I guess you can only use it once.
If your brother doesn't use it anymore, see if he still has the iLok USB that should be with it. Most authorized softwares need them to run, to stop counterfeit software.